Portable Document Format

Portable Document Format (PDF) is a file format developed by Adobe Systems for representing documents in a manner that is independent of the original application software, hardware, and operating system used to create those documents. A PDF file can describe documents containing any combination of text, graphics, and images in a device independent and resolution independent format. These documents can be one page or thousands of pages, very simple or extremely complex with a rich use of fonts, graphics, colour, and images.

Free readers for many platforms are available for download from the Adobe website.

Several independent PDF viewers and interfacing libraries have been developed, for example Xpdf, and GNOME Pdf for POSIX-like systems.

Table of contents
1 Technology
2 Comparison between reading a PDF file and a HTML file
3 See also
4 External Links

Technology

PDF is primarly the combination of three technologies:

  • a cut-down form of PostScript for generating the layout and graphics,
  • a font-embedding/replacement system to allow fonts to travel with the documents, and
  • a structured storage system to bundle these elements into a single file, with data compression where appropriate

Postscript

PostScript is computer language -- or more accurately a page description language -- that is run in an interpreter to generate an image. This is a complex process that requires a fair amount of resources in order to work.

PDF is a subset of those PostScript language elements that define the graphics, and only requires a very simple interpreter. For instance, flow control commands like if and while are removed, while graphics commands such as lineto remain.

That means that the process of turning PDF back into a graphic is a matter of simply reading the description, rather than running a program in the PS interpreter. However the entire PS world in terms of fonts, layout and measurement remains intact.

Often the PostScript-like PDF code is generated from a source PostScript file. The graphics commands that the PS code outputs are collected and tokenized, any files, graphics or fonts the document references are also collected, and then everything is compressed into a single file.

Advantages

There are several advantages to the PDF format. One is that there is only a single small file to transfer, whereas with the same file in PostScript format one must send the additional materials on their own. In addition the PostScript code is already interpreted, so it is faster to display on the screen. Finally, if displayed with Adobe's Acrobat Reader, there is a font-substitution strategy that ensures the document will be readable even if the end-user does not have the "proper" fonts installed.

History

When PDF first came out, in the early 1990s, it was slow to catch on. At the time, not only did the only PDF creation tools of the time (Acrobat) cost money, but so did the software to view and print PDF files. Additionally, there were competing formats. Adobe started distributing the Acrobat Reader program at no cost, and continued to support PDF through its slow multi-year ramp-up. Competing formats eventually died out, and PDF became a well-accepted standard.

Macintosh

PDF was selected as the "native" metafile format for Mac OS X, replacing the PICT format of the earlier Mac OS. Mac OS X's imaging model, Quartz, is closely based on the Display PostScript standard, and is thus highly compatible with PDF. Because of the OS support, all OS X applications can create PDF documents automatically as long as they support the Print command.

Comparison between reading a PDF file and a HTML file

PDF and HTML are not equivalent technologies, but are both commonly found on the web.

HTML vs. PDF

HTML is a method for describing the content of a web page in a manner that is open to interpretation by the browser which renders it on the user's screen. This permits content to be rendered to suit the viewer rather than the content provider. PDF on the other hand is strictly concerned with describing the content of a document such that the original intention of its author is fully preserved.

Difference

Since many content providers find the fluid nature of HTML rendering counter to their desires in building a web page, PDF has become widespread as an alternative method of presentation. Other workarounds within the HTML domain have also become commonplace, such as using tiled GIF images in an attempt to force a particular layout.

A typical example of the differences this leads to for the viewer is with zooming: enlarging a PDF document magnifies the text but preserves the original layout and spacing - with HTML a larger font size is used and lines re-wrap accordingly to fit the user's available browser window. Such differences in behaviour follow naturally from the very different approaches the two formats take to content rendition.

See also

External Links


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